As the semiconductor technology develops, an integrated circuit often contains some devices operating at a high voltage level, and other devices operating at a low voltage level. The low voltage devices may not tolerate a high voltage signal. Device failures happen frequently, when the low voltage devices operate with the high voltage signal. In order to protect the low voltage devices from the high voltage signals, the integrated circuit often includes an input/output (I/O) circuit as an interface between the low voltage devices and the high voltage devices. The I/O circuit allows the low voltage devices communicating with the high voltage devices, while protecting the low voltage devices from interfered by the high voltage signals.